Jacques P. Vallée

Abstract. The observational study of galactic magnetic fields
dates back to 1949; an
excellent review of the early 30 years has been made by
Verschuur (1979).
I review here the developments since then and the current state of our
observational knowledge on the magnetic
fields inside and outside the Milky Way galaxy, for objects with
sizes greater than 1 parsec (= 3.2
light-years; = 3.1 × 1016 m). Included are the medium-scale
magnetic fields in the isolated
globules, dusty elongated clouds and narrow filaments, large
interstellar superbubbles, and
in large-scale magnetic fields in the spiral arms in our Galaxy
and in objects outside our
Galaxy out to cosmological distances. The large-scale magnetic
fields can act as guides to
the low density gas in its motion in the rarefied areas of the
interstellar medium, and as
tracers of the past dynamical histories of galaxies in motion,
linking galactic dynamic
with galactic dynamos. Medium-scale magnetic fields can play a
support role, supporting
clouds against outside pressures or against collapse due to
self-gravity. Small-scale magnetic
fields play a significant role on smaller-scale phenomena:
propagation of cosmic-rays, shock
waves, cosmic dust orientation, star formation (although there is
little detailed discussion here of
magnetic fields on star formation and objects with sizes < 1
parsec).